1. Field of Invention
The present invention relates to protection of wires and cables used in buildings and more particularly to fire and thermal protection of the wires and cables.
2. Description of Prior Art
Wire and cable materials installed in buildings do not represent a major quantity of the flammable material (fire load) in a building. However because they are installed concealed in ceilings, floors and walls connected via shafts and raceways they do present a major hazard to persons and equipment. During a fire event these shafts and raceways provide for ready transport of flame, smoke and toxic and corrosive gases throughout a building. It is for this reason that the National Electrical Code (NEC) sets requirements that limit the flame spread and combustion gases from burning were and cables in buildings.
Polyolefin resins, particularly polyethylene and polypropylene, are superior materials for wire and cable building application in all respects except flammability. Technology to add flame retardance, involving both halogen additives and non-halogen additives, has been developed. When used to improve flammability in polyolefins serious compromises in the overall performance characteristics of the system result.
The disadvantages of the halogen additive approach are reduced electrical performance of the material and increased smoke and toxic and corrosive gases on combustion. Moreover, even when employed at relatively low levels, flame retardants significantly add to compound cost.
The non-halogen additive approach also reduces electrical performance but does not compromise the combustion advantages of polyolefin resins while imparting flame retardance. However because significant flame retardance is only aired through the addition of high levels of metal salts, such as, aluminum and magnesium hydrates, the resultant formulated products have higher costs, process more slowly and have somewhat reduced physical and mechanical properties when compared with the original non-flame retarded polyolefin base resin.
The overall effect of adding flame retardants to polyolefins is to limit use, as an insulation, to low voltage electrical power, no voice or data transmission, and to intermediate level, flame retardant (FR) jackets. In both areas FR polyolefins compete with PVC, a lower cost inherently flame retarded material. Neither PVC nor FR polyolefin compound provide for thermal protection of covered wires.
There are a number of cases where thermal protection for wires is disclosed. In U.S. Pat. No. 4,822,659, to Anderson, et. al., a wrap or fire block sheet based on a preformed silicone foamed layer, containing aluminum trihydrate to provide fire dance, is bonded to a non-flammable glass cloth. The disadvantages to this approach, particularly in wire and cable application, is the added process step needed to disperse a mineral filler throughout a reactive mixture and the subsequent need to foam and adhere the foam to the supportive substrate. This foam barrier will increase the thickness of the construction; a distinct disadvantage in building wire where space allotted for installation is limited. Moreover, the silicone raw materials and the platinum catalyst add significantly to the overall cost for the application. U.S. Pat. No. 5,202,186, to Williamson, discloses a thermal protective sleeve consisting of a sandwich structure of a laminate of silicone foam provided on both sides of a glass state. The deficiencies here are those cited for the foam in U.S. Pat. No. 4,822,659 plus the lack of adequate fire protection, absent the aluminum hydrate filler. U.S. Pat. No. 6,037,546, to Moltine et. al., discloses the use of a heat/flame resistant layer from a foamed thermoplastic polyvinylidene fluoride (PVDF) material. This technology reports to provide thermal and fire protection but has the disadvantage of adding significant levels of toxic and corrosive acid combustion gases in a fire event. In addition foaming the PVDF, either by gas injection or chemically, adds complexity and cost to the manufactured article. High raw material cost is generally associated with all fluorocarbon resin systems.
Solid fluorocarbon resins are used in wire and cable application when high cost is not the prohibitive factor, as for example, in plenum cable application. Flourinated ethylene propylene resin (FEP), is used where superior electrical properties, similar to those possessed by polyolefins, are required. FEP is chosen based on the fire resistance despite the disadvantages of high cost, a consistent vagrant supply situation and the potential for toxic combustion gases. These serious disadvantages of FEP create the need for a fire and heat resistant polyolefin system in those areas demanding superior electrical performance.
Where cost is not the controlling factor, polyvinylidene difluoride (PVDF) is the fluorocarbon resin of choice for application as a FR jacket. PVDF provides mechanical and fire protection for cables used in buildings including plenum application. The disadvantages of PVDF are its high cost and the noxious combustion by-products. PVDF decomposes thermally to produce copious quantities of the highly toxic and corrosive acid gas, hydrogen fluoride. This acid is recognized for its corrosive action on metals and glass fibers. Finally, solid PVDF used as a jacket material does not provide thermal protection to the wires or cables beneath the covering.
Polyvinyl chloride (PVC) is a lower cost inherently flame retardant polymer used in building wires and cables. PVC's are similar in electrical performance to flame retard polyolefins. The polarity of PVC formulations limit use as an insulation material to electrical signal low voltage, for example, non-metallic building wire (NM-B), application. However the poor electricals do not prevent use as flame retardant jacket material. Relative to fluorocarbons the lower cost is a very attractive feature. The drawback with PVC jackets is the noxious by-product combination gases. The first step thermal decomposition of PVC produces substantial quantities (>50% by weight) of the toxic, strongly corrosive acid, hydrogen chloride. Further, on combustion, PVC produces dense black smoke. These features are in sharp contrast with the low smoke and low acid gas produced from the combustion decomposition of non-halogen flame retardant polyolefins. Finally, PVC jackets do not provide thermal protection to the wires or cables covered.